The girders carrying the upper flights spring from the landing girders, and have their upper ends bolted on to the main girders supporting the galleries, which are varied in pattern for this purpose. The railing of the staircase is formed in separate cast-iron standards, one to each step, which are bolted on to the top flange of the girders; and the foot of the standard is so continued that the ends of the treads are fitted into it, and are thus supported. The pattern of these standards is assimilated to that of the gallery railing.
The hand-rail is formed of Honduras mahogany, with carved ends. On each side of the upper flight, which occupies the centre of a 24-feet space, connecting-galleries about eight feet wide are carried, establishing a communication between the two lines of gallery without descending to the level of the landing and then re-ascending. The landing is sufficiently high above the ground-floor to give ample headway for passing underneath it; so that the space occupied by the staircases on the ground-floor is but small.
T now only remains to mention briefly the construction of the floor of the building, and the foundations for the base-pieces. The substratum of the site consists of gravel of an excellent quality, and sufficiently dense to have sustained, perhaps without any preparation, the load brought upon it by the bases of the columns. A thickness of concrete, proportioned in all cases to the amount of the weight to be borne by the superincumbent columns, and of such a size as to be two feet in each direction larger than the bed-plates, was placed upon the gravel, and the upper surface was finished with a bed of fine mortar to receive the bed-plates. In this manner it was calculated that in no case would a greater weight than two-and-a-half tons be borne by each foot superficial of the gravel—previous experiments having shown that a considerably larger weight could be placed upon it without any injurious effect.
The timbers supporting the joists for the floor are also placed upon small blocks of concrete, about one foot cube, at a distance of eight feet apart. On these are fixed the flooring-joists, and a deal floor an inch and a half thick is laid on them, as has been already mentioned, with intervals of about half an inch between the boards.
FIXING CAST-IRON DRAIN-PIPE.
In order to carry off the water brought down from the roof by every alternate longitudinal row of columns, 6-inch cast-iron pipes are fitted into the sockets described in the base-pieces, and are carried in the lines of those columns through the whole length of the building, with discharges into the larger drains at the centre and at each end; the natural slope of the ground gives a sufficient fall to the pipes.
Having thus described in detail all the different portions of the construction of the building, we must proceed to give some account of its actual erection, which will enable us to mention many very ingenious mechanical contrivances which were employed in the course of its progress.